This study will have two groups of patients with CKD and one group of patients that have normal kidney function, but all groups will have low levels of vitamin D. The two groups with CKD (group 1 and group 2) will receive either cholecalciferol or calcitriol. The purpose of having a control group (group 3) without CKD will be to evaluate if any changes that are witnessed in response to vitamin D therapy are specific to patients with kidney disease or apply to all patients with vitamin D deficiency who receive vitamin D supplements.